Monitoring synchronization of a motor using stator current measurements

Abstract

Monitoring synchronization of an electric motor using current signals from power supplied to the motor is disclosed herein. The current signals may be used to calculate representative current values which may be used to calculate a rotational frequency of the rotor of the motor. The rotational frequency may be used to determine synchronization, such as slip. Monitoring may be during startup of a synchronous motor in induction mode. Upon reaching a predetermined synchronization threshold, the motor may be configured from induction mode to synchronous mode.

What is claimed is: 
     
       1. An intelligent electronic device (“IED”) for monitoring synchronization of a motor using stator currents, comprising:
 an input in communication with current transformers (“CTs”) each in electrical communication with a phase of current supplied to stator windings of the motor; 
 a processor in communication with the input, receiving current data points corresponding with the current supplied to the stator windings; and, 
 a non-transitory computer-readable storage medium in communication with the processor, the non-transitory computer-readable storage medium comprising:
 a synchronization module configured to
 calculate a root-mean squared value from the current data points for each phase of current supplied to the stator windings of the motor; 
 sum the root-mean squared values from each phase; 
 calculate a DC component from the sum of the root-mean squared values from each phase; 
 calculate representative current values by subtracting the DC component from the sum of the root-mean squared values; and 
 calculate using the representative current, and without using values from a voltage across a discharge resistor of field windings of the motor, an estimate of synchronization of
 a rotational frequency of the rotor 
 a rotational frequency of an electric field of the stator windings of the motor; and 
 
 control a configuration of the motor based on the estimate of synchronization by opening a switch to disconnect the discharge resistor from the field windings of the motor when the estimate of synchronization has reached a predetermined threshold. 
 
 
 
     
     
       2. The IED of  claim 1 , wherein the synchronization comprises one selected from the group consisting of: slip; and, a ratio of a rotational frequency of a rotor of the motor to a nominal frequency of the motor. 
     
     
       3. The IED of  claim 1 , wherein the motor comprises one selected from the group consisting of: a synchronous motor; and, an induction motor. 
     
     
       4. The IED of  claim 1 , wherein the synchronization module is further configured to:
 determine zero crossings using the representative current values; and, 
 estimate a frequency of a rotor using the zero crossings. 
 
     
     
       5. The IED of  claim 1 , wherein the motor comprises a synchronous motor started in induction mode, and the synchronization module is further configured to reconfigure the motor from induction mode to synchronous mode when the synchronization reaches a predetermined threshold. 
     
     
       6. The IED of  claim 1 , wherein the synchronization module is further configured to calculate the estimate of synchronization without using values from a voltage of the field windings of the motor. 
     
     
       7. The IED of  claim 6 , wherein the synchronization module is further configured to calculate the estimate of synchronization without using voltage values. 
     
     
       8. The IED of  claim 1 , wherein the synchronization module is further configured to calculate the estimate of synchronization without using values from the field windings of the motor. 
     
     
       9. An intelligent electronic device (“IED”) for monitoring synchronization of a motor using stator currents, comprising:
 an input in communication with current transformers (“CTs”) each in electrical communication with a phase of current supplied to stator windings of the motor; 
 a processor in communication with the input, receiving current data points corresponding with the current supplied to the stator windings; and, 
 a non-transitory computer-readable storage medium in communication with the processor, the non-transitory computer-readable storage medium comprising:
 a synchronization module configured to:
 calculate a root-mean squared value from the current data points for each phase of current supplied to the stator windings of the motor; 
 sum the root-mean squared values from each phase; 
 calculate a DC component from the sum of the root-mean squared values from each phase; 
 calculate representative current values by subtracting the DC component from the sum of the root-mean squared values; 
 calculate using the representative current values, and without using values from a voltage across a discharge resistor of field windings of the motor, an estimate of synchronization of a rotational frequency of the rotor and a nominal frequency of the stator windings of the motor; and, 
 reconfigure the motor from induction mode to synchronous mode when the synchronization reaches a predetermined threshold by signaling a switch to close to connect a power source to the field windings of the motor. 
 
 
 
     
     
       10. An intelligent electronic device (“IED”) for monitoring synchronization of a motor using stator currents, comprising:
 an input in communication with current transformers (“CTs”) each in electrical communication with a phase of current supplied to stator windings of the motor; 
 a processor in communication with the input, receiving current data points corresponding with the current supplied to the stator windings; and, 
 a non-transitory computer-readable storage medium in communication with the processor, the non-transitory computer-readable storage medium comprising:
 a synchronization module configured to:
 calculate root-mean squared current values from the plurality of current values from each phase; 
 sum the root-mean squared current values from each phase; 
 calculate a DC component from the sum of the root-mean squared current values; 
 calculate representative current values by subtracting the DC component from the sum to produce representative current values; 
 calculate, using the representative current values and without using values from a voltage across a discharge resistor of field windings of the motor, an estimate of synchronization of a rotational frequency of the rotor and a nominal frequency of the stator windings of the motor; and 
 control a configuration of the motor based on the estimate of synchronization by signaling a switch to open to disconnect the discharge resistor from the field windings of the motor when the estimate of synchronization has reached a predetermined threshold. 
 
 
 
     
     
       11. The IED of  claim 10 , wherein the synchronization module is further configured to reconfigure the motor from induction mode to synchronous mode when the synchronization reaches a predetermined threshold.